To meet the bandwidth requirements of current and future high-speed applications, state-of-the-art telecommunication components and systems must provide a host of sophisticated signal processing and routing functions, both in the optical and electronic domains. As the complexity level increases, integration of more functions and components within a single package is required to meet system-level requirements and reduce the associated size and cost of the end system. It has been recognized for some time that the integrated circuit devices, processes and techniques that revolutionized the electronics industry can be adapted to produce optoelectronic integrated circuits. In typical optoelectronic integrated circuits, light propagates through waveguides of high-refractive-index materials such as silicon, gallium arsenide, lithium niobate, or indium phosphide. The use of high-index materials enables smaller size devices, as a higher degree of mode confinement and tighter bends may be accommodated. While all transmitter, signal processing and receiver functions may be incorporated in a single optoelectronic integrated circuit, the system may also be constructed from more than one package, in what is defined herein as “multi-module integration ”. Multi-module optoelectronic integration is required when a desired component is incompatible with the integrated device. In some cases, no method of fabricating the component is compatible with the technology being considered, while in others it may be impossible to realize the full set of specifications for the component in the integration platform. Even if all the components are readily achievable in a single integrated device, multi-module optoelectronic integration will still be required to meet all user needs. For example, the end user may require only a limited subset of the transmit/process/receive functions, or the user may desire specific or unique physical input and output signal configurations.
Multi-module optoelectronic integration is crucial to the success of silicon-based optoelectronic integrated circuits. A clear advantage of using silicon-based optoelectronic integrated circuits stems from the fact that many required tools, techniques, and processes have already been developed in silicon to meet the needs of conventional electronics. In addition, the material costs of silicon-based devices are considerably lower than those for competing technologies such as gallium arsenide or indium phosphide. However, since silicon-based lasers are just beginning to be developed, it is not currently possible to incorporate the light source in the same silicon wafer as the signal processing and receiver elements. Thus, the light signal must be introduced to the silicon waveguide from an external source.
One common external source is a separate laser module emitting a free space beam, followed by optical elements to shape, focus, and steer the beam, or adjust its polarization state. A second common external source configuration consists of a fiber-connected (referred to in the art as “pigtailed ”) laser module or another light signal delivered through an optical fiber, again followed by a similar train of optical elements. While receiving elements may be incorporated in the silicon wafer as on-chip or integrated detectors, there are many applications where the user will need direct access to the optical signal after the on-chip functions have been performed. Thus it is appropriate to provide an optical output port that would generally be a fiber-based termination, although the preferred embodiments do not exclude other output configurations.
A common prior art technique for coupling light from an external source to a silicon waveguide is to cleave end facets on both the waveguide and the mating fiber termination. Examples of fiber terminations include, but are not limited to: multimode or single-mode fibers with small or zero cleave angles, and specially-shaped or lensed single-mode fibers that produce spot sizes as small as 1.5 μm. The fiber termination is aligned to allow maximum light transmission through the waveguide, and then fixed in position. Anti-reflection (AR) coatings can be used on both the fiber termination and the waveguide facet to reduce the Fresnel losses. Since input and output ports for devices must be located at edge facets of the waveguide-containing wafer die, significant restrictions on device geometry (e.g., topology and/or size) are imposed by using this prior art edge coupling constraint.
The above-described edge coupling technique is effective if the mode-field diameter of the desired mode in the waveguide is similar to the spot size associated with the fiber termination, and if the numerical apertures (NAs) of the fiber termination and waveguide are well-matched. However, in many practical applications, silicon waveguides must be relatively thin, having a thicknesses of less than 0.35 μm (with a numerical aperture (NA) essentially equal to 1) to remain single-mode in the vertical direction and enable high-speed electronic applications. By way of comparison, single mode fibers that are commonly used for telecommunications applications have mode field diameters ranging from 2.5–10 μm, with NAs ranging from 0.1–0 4. Therefore, it is clear that this edge coupling technique is not readily applicable for use with relatively thin, sub-micron single mode silicon waveguides.
As direct coupling in the above-described manner does not provide a sufficiently small spot size, alternative techniques to transfer light into the waveguide must be used. In one prior art technique, light is incident on a periodic grating structure that may be fabricated through conventional lithographic techniques. See, for example, Fundamentals of Optoelectronics, Chicago, Richard D. Irwin, Inc., by C. Pollock, 1995, at pages 309–320.
In a second prior art technique, the beam is incident upon an optical element of high-index material that is disposed in very close proximity to the waveguide of interest. One exemplary arrangement of this technique is disclosed in an article entitled “Theory of Prism-Film Coupler and Thin-Film Light Guides” by P. K. Tien et al., appearing in the Journal of the Optical Society of America, Vol. 60, 1970 at pages 1325–1337. In this context, “very close proximity ” is intended to mean that the separation distance between the optical element and the waveguide permits evanescent coupling of light from the optical element to the waveguide. In order for evanescent coupling to occur, the medium separating the optical element from the waveguide must have a refractive index that is lower than those associated with the optical element and waveguide materials. In addition, the refractive index of the launch optical element must equal or exceed that of the waveguide material. In order to couple light efficiently from the optical element to the waveguide for a specified wavelength and waveguide thickness, light must be incident on the waveguide at a specific angle of incidence. To readily achieve the required angle of incidence, the optical element is frequently fabricated in the form of a prism. By varying the angle of incidence of the external beam on the angled facet of the prism, the beam inside the prism can be refracted at the desired angle. For this reason, the evanescent technique is generally referred to in the art as “prism coupling ”.
While coupling light into and out of a thin waveguide by launching light into and retrieving light from a high-index prism or prisms in close proximity to the waveguide layer is a well-established technique in laboratory and waveguide characterization applications, the physical connection of the prisms to the waveguide is temporary and not appropriate for a finished device that will be subjected to typical user conditions. Several innovative developments are required before prism coupling techniques can be effectively utilized to couple a significant amount of light into and out of a device prototype or a finished product.
For example, methods of physically attaching prisms and waveguides to produce dimensionally stable interfaces with both a high degree of mechanical integrity and sufficient optical transmission are virtually undeveloped. Another key element is to identify and develop appropriate materials and mass production techniques for prism structures. Each prism element must be visually inspected and optically tested to verify that it meets the stringent optical and mechanical specifications required by most prism coupling applications. Thus, current methods of prism fabrication cannot be readily transferred to efficient production of device structures requiring multiple input and output ports. Moreover, the high-index material prisms (for example, titanium dioxide—rutile) used in the prior art for waveguide evaluation cannot be used to evanescently couple light into silicon waveguides due to the refractive index mismatch. Additionally, manufacturable designs and methods of producing precision prism structures, and appropriate methods of maintaining the required set of geometrical constraints governing the prism, evanescent coupling layer, waveguide, and input and output beams over a typical device lifetime remain lacking in the prior art.
Thus, a need remains in the art for a robust technique for evanescently coupling light into and out of thin silicon waveguides, with the coupling arrangement being permanently attached to the optoelectronic circuit.